Abstract / Summary
In 2023, tuberculosis (TB) once again became the world's deadliest infectious disease, overtaking COVID-19 as the leading cause of death from a single infectious agent, Mycobacterium tuberculosis (MTB). The World Health Organization (WHO) reports that the COVID-19 pandemic has reversed years of progress in providing essential TB services and reducing TB disease burden. In 2025, TB was responsible for an estimated 1.23 million deaths globally (95% uncertainty interval: 1.13–1.33 million), and there were 400 K cases of rifamycin resistant/multidrug resistant TB (RR, RIFR, MDR). While treatment success for drug susceptible TB remains high (88%), outcomes for MDR/RR-TB are lower but improving, reaching 71% globally. Clearly, there is a critical unmet need for novel antibiotics against MTB, exacerbated now due to COVID-19. The limited success in pursuing novel targets underscores the urgent need to revisit and reevaluate the mechanisms of action behind existing and once effective drugs, a strategy that may hold untapped potential in the fight against growing antibiotic-resistant TB. The MTB RNA polymerase (RNAP) is a proven target for TB treatment by rifampin (RMP), which kills both active and latent MTB. However, RMP suffers from several drawbacks including: long treatment times, activation of hPXR (inducing Cyp3A4 which is problematic for HIV-TB coinfection), and resistant mutations within RNAP. Two newer treatments with novel mechanisms of action include bedaquiline and delamanid, the former has been approved by the FDA for the treatment of multiple-drug resistant tuberculosis (MDR-MTB); however, even these new drugs already have incidences of acquired resistance. Our goal is to develop novel drug candidates for TB treatment that: target RNAP, have the potential to shorten treatment times, and have activity against RIFR mutants. Our approach, orthogonal to the rifamycins, is to target the interaction of MTB RNAP with the required transcription factor CarD. We have previously labeled CarD with the BODIPY-FL fluorophore via engineered cysteine mutations and developed a fluorescence polarization assay to monitor its interaction with RNAP and promoter DNA. Leveraging this target, we screened a 24 K small molecule library against CarD binding to MTB RNAP. We have now extended this screening to∼88,000 additional molecules.